Maybe if you attach a Glock to it the AR will absorb some of the Glock's reliability. I'm thinking of duct taping a Glock to my car engine to make it run more reliably.
That was a joke. I really can't see the advantage of a pistol attachment. why not just carry the pistol and change weapons if needed? Maybe it is faster to just move your hand? I think the more practical thing would be to just get a drum magazine for the AR if you need some extra shots in a pinch.